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X i v :q u a n t -p h /0008118v 1 28 A u g 2000
Appl.Phys.B manuscript No.(will be inrted by the editor)
Applications of Integrated Magnetic Microtraps
J.Reichel ⋆,W.H¨a nl,P.Hommelhoff,T.W.H¨a nsch
Max-Planck-Institut f¨u r Quantenoptik and Sektion Physik der Ludwig-Maximilians-Universit¨a t Schellingstr.4,D-80799M¨u nchen,Germany Submitted 24.07.2000,revid version 25.08.2000
Abstract Lithographically fabricated circuit patterns can provide magnetic guides and microtraps for cold neu-tral atoms.By combining veral such structures on the same ceramic substrate,we have reali
zed the first “atom chips”that permit complex manipulations of ultracold trapped atoms or de Broglie wavepackets.We show how to design magnetic potentials from simple conductor pat-terns and we describe an efficient trap loading procedure in detail.Applying the design guide,we describe some new microtrap potentials,including a trap which reaches the Lamb-Dicke regime for rubidium atoms in all three dimensions,and a rotatable Ioffe-Pritchard trap,which we also demonstrate experimentally.Finally,we demon-strate a device allowing independent linear positioning of two atomic clouds which are very tightly confined lat-erally.This device is well suited for the study of one-dimensional collisions.
⋆
Fax:+4989285192;ichel@physik.uni-muenchen.de
[4].Such complex potentials can be realized when litho-graphic or other surface-patterning process are ud to produce the field-creating structures on a suitable sub-strate,now sometimes called an “atom chip”.The u of lithographic,planar conductor patterns for magnetic atom trapping has been propod as early as 1995[5].However,no successful experiments were carried out at that time due to the difficulty of loading such traps,which have small volumes and are typically located onl
y a few hundred micrometers or less from the substrate surface.This situation changed last year with our demon-stration of an efficient loading mechanism for surface traps [6].It employs a novel mirror-MOT,using a re-flecting layer on top of the circuit pattern to realize the lar fields for lar cooling and trapping in clo prox-imity to the surface.With this loading mechanism,inte-grated traps for cold atoms became experimentally ac-cessible.In the first demonstration of such a trap,the new loading mechanism was employed to fill a minia-ture quadrupole trap featuring transver gradients of 1700G /cm [6].The trapping potential was created by a lithographically produced,U-shaped conductor in con-junction with an external bias field.In the meantime,the same loading and trapping techniques have also been applied to construct a Ioffe-Pritchard (IP)trap for 7Li atoms [7].The mirror-MOT technique has also proven its ufulness beyond lithographic traps as it was ud to fill surface traps of a different type,which result from the combination of permanent surface fields on magnetic tape with an external bias field [8].
Indeed,lithographic conductors are now increasingly employed to trap,guide and manipulate cold neutral atoms.Parallel conductors have been ud to realize atom guides in which atoms are confined in two dimen-sions and move freely along the third [9,10].Very re-cently,more complex conductor patterns were ud to re-alize an atomic conveyer belt,which adiabatically trans-ports ato
ms and positions them with a precision of the order of 1µm [11].Moreover,two “beam splitters”have been demonstrated,one which distributes a trapped cloud
2J.Reichel et al.
in a Y-shaped pattern [12],and
one
which
splits
a
guided beam
using an
X-shaped
pattern [13].Such devices demon-strate the versatility of the new approach.Considering the relative ea of the experiments now that major obstacles have been removed,we foree a wide variety of applications for lithographic microtraps.In this arti-cle,we discuss some key issues of microtrap design and realization.We propo a modular,intuitive approach to the design of complex potentials from a few simple building blocks.In the following ctions,we tackle the substrate technology and give a detailed description of the loading procedure.The wide-ranging possibilities of-fered by simple conductor configurations are illustrated by experimental results of trapping in some fundamental types of potentials which would be very difficult to cre-ate by more traditional means.Finally,we demonstrate a linear collider for trapped atoms,which is well suited for the study of one-dimensional collisions.
2Design of integrated microtraps
In this ction we prent a method to design a variety
of microtraps from simple 2D-conductor configurations which rve as modular building blocks.As central part we investigate the magnetic potential at a perpendicular wire interction,which can then be ud to construct more complex magnetic potentials.Throughout the dis-cussion we will denote e
z the vector normal to the sub-strate surface,all current carrying wires are contained within the xy plane at z =0.2.12D-quadrupole fields
Ioffe-Pritchard (IP)traps can be regarded as superposi-tion of a two-dimensional quadrupole field for transver confinement and a longitudinally varying field for con-finement along the quadrupole axis.The 2D-quadrupole field can easily be created in the vicinity of a current carrying wire (here I 0along e x ),if its tangential field B =µ0R is compensated at the point r =(x,0,z 0)by the homogeneous field
B =B 0,y e y =
2G
µ0
B 20,y
b
,
˜B z b
,z 0+
˜B
y b
2,
˜B
z 2π
成都会议I z 0
2π
I x 1
The parameters are realistic,e Sect.3.1
Applications of Integrated Magnetic Microtraps
3
Fig.2The field strength in the center of the 2D-quadrupole can be modified by a current crossing the first wire perpen-dicularly.The x component of its magnetic field determines the longitudinal trap potential (bottom),the y component displaces the transver field minimum (solid line)from the the original quadrupol line (dashed)in the xy plane.
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50
75
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50
75
-20
0204
la时间2
0-2-4
B x [G]
B y [G]
x [µm]
x [µm]
y min [µm]
花图片(a)
(b)
Fig.3Field contributions of an intercting wire at height z 0=25µm for a current of I 1=0.5A.
which are shown in fig.3.
As the lorentzian shape of B w x suggests,the field con-tribution of the intercting wire can be ud to create a repulsive potential along the quadrupol axis (Fig.4a-c).However,the repulsive character of the crossing current can be converted into an attractive one if a homoge-neous bias field B 0,x <−B w
x (x =0,z 0)is superimpod.In this ca,the field along the axis becomes
B min (x )≈ ˜B x (x,y =0,z 0) =|B 0,x |−|B w x (x,z 0)|(9)which exhibits a minimum at x =0.This field config-uration thus provides a trapping potential in all three
dimensions (fig.4d-f).Wires supporting a current flow perpendicular to a quadrupole guide can therefore be ud to create repulsive as well as attractive potentials along the trap axis,depending on the strength of the external field B 0,x .
In order to reflect the physical properties of the trap-ping potentials,each plot in fig.4shows the minimum potential value within the plane (a,d)or line (b,c,e,f)that is perpendicular to the visible line or plane.
A detailed look at the potentials in fig.4reveals how the minimum position of the trap is displaced by the男人的占有欲
field component B w
y of the crossing current.The approx-imations in eq.4and 5hold as long as the intercting current is smaller than the current in the central wire (I 1≪I 0).Here we have chon I 1=1
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al.
Fig.5Field strength at two neighbouring wire interctions.The distance of the crossing wires has been chon d =4z 0=100µm,parameters are I 0=2A,I 1=I 2=0.5A,B 0,y =160G.B 0,x is chon as in fig.4.
10
3050B min (x) [G]x [µm]10
3050B min (x) [G]
x [µm]
I 1
I 3I 2I 1I 3I 2a)
b)
Fig.6Longitudinal trapping field for B 0,y =160G.a)The H-shaped IP-trap with a =z 0is optimized for maximum field curvature along the slow axis.b)An oppod current in the center of the to intercting wires allows a further increa of the longitudinal field curvature and trapping frequency.The modulus of current I 2must be smaller than I 1+I 3
Lamb-Dicke
dir.osc.freq.ν142kHz
1.09·109G/cm 2
0.30
3
5.1kHz
Applications of Integrated Magnetic Microtraps 5
to create smaller structures and leads to a very smooth gold surface.
The mirror-MOT requires a reflective substrate sur-face.As the conductor pattern forms a relief structure on the surface,directly applying a coating to it would lead to a poor mirror with strong diffusion from the conductor edges.We therefore u a
simple “replica optics”proce-dure as described in figure 7to obtain a flat mirror.The final substrate has the layer structure shown in figure 8.The size of our substrates is 22.4mm ×18.4mm.This is large
enough even for complicated conductor patterns,but limits the size of the mirror and thus the MOT beam diameter,e ction 4.Thin-film hybrid substrates are available in larger 2in ×2in.
The maximum sustainable current I through the litho-graphic conducters is a key parameter in order to obtain steep traps.The thinnest wires on our hybrid substrates have a cross ction of h =7µm height and w =10µm width.With the literature value of ρ=2.2µΩcm for the specific resistance of gold,this leads to a resistance of 3.1Ω/cm and a dissipated power of P =3.1W /cm at I =1A.Two effects impo limitations on I :global heating of the substrate by this considerable total power,and local effects which lead to melting or evaporation of the wire at one specific point.The first limitation can always be alleviated by improving the heat conductivity from the substrate to the rervoir to which it is con-nected,or,if necessary,by cooling this rervoir (e.g.,by using a small liquid nitrogen tank).The cond,lo-cal effect is characteristic of the microfabrication process and materials and is the one which actually limits I .In our tup,active cooling proved to be unneces-sary and the cooling
system is most simple.Heat re-moval from the substrate is ensured by fixing it on a copper block with an UHV-compatible,thermally con-ductive epoxy resin (Epo-Tek H77).The copper block acts as a heat buffer and is itlf connected to the stain-less steel vacuum system by copper rods.We have de-termined the maximum sustainable currents in hybrid wires (h =7µm,w =10µ,length l =20mm)in this asmbly.The tests were done in air with the copper block pod on a wooden table and initially at room temperature.We obtained the following result:currents of I =3A could repeatably be applied for one minute;the copper block heated up by about 40K during that period.At currents above I =3.2A,some wires blew up,forming ∼10µm long gaps in the wire.Thus,the highest sustainable current density (at I =3A)was j =4.6·106A /cm 2.The calculated magnetic field of a 10µm wide conductor carrying a current of I =3A is 425G at a distance d =10µm from the surface and has a gradient of b =4.12·105G/cm.Superposing a suffi-cient longitudinal bias field to suppress Majorana loss (ωosc ≈10ωprec ,[17]),a Ioffe-Pritchard trap with a transver oscillation frequency of 270kHz could be re-alized for 87Rb atoms.The resulting confinement leads to a Lamb-Dicke parameter of 0.12with respect to the
Fig.7Steps of the replica technique leading to a smooth mirror surface.(a)A small amount of UHV-compatible epoxy glue (Epo-Tek 353ND)is dispend on the hybrid substrate;a 250nm silver layer (w
hich will rve as the mirror)has previously been sputtered onto an intermediate substrate.(b)The intermediate substrate is sandwiched onto the hy-brid substrate,with the silver layer facing the epoxy.Short stretches of 25µm-diameter gold wire rve as spacers.(c)After curing the epoxy,the intermediate substrate is lifted off,leaving the silver mirror layer on the epoxy.
300 µ25 µFig.8Layer structure of the microtrap substrate.
D2line,corresponding to a ground state 1/e 2diame-ter of 60nm.The surprising figures suggest that traps with exciting new properties may be realized with this simple technique.
3.2Vacuum system and outgassing issues
The conductors on the substrate are contacted to con-nector pins by gold wire bonding.This technique,ex-tensively ud in microchip packaging,requires no sol-dering compounds and is therefore well suited for UHV u.However,a special wire bonder is required.Con-tacting with a silver-filled epoxy is an alternative if a wire bonder is not available [7].Kapton isolated copper wires with matching UHV connectors are ud between the connector pins and the vacuum feedthrough.
基建工程
Considering the quantity of materials employed in vacuo ,the final pressure in the system is of intere
st.Our small-volume vacuum system is pumped by a 25l /s ion pump and a simple titanium sublimator.Both pumps are connected to the glass cell containing the substrate by 35mm stainless steel tubing,involving veral 90◦angles
6J.Reichel et al.
Fig.9Geometry of the experiment.The substrated is mounted upside down in the vacuum chamber,so that the atoms are trapped below the surface.This enables time-of-flight imaging to measure the velocity distribution.The probe beam is directed parallel to the surface along the y axis. between the cell and the pumps.A vacuum meter,which is located about midways between the pumps and the glass cell,indicates afinal pressure of3·10−10mbar typ. after3days of baking at140◦C.
More importantly,we obrve1/e lifetimes in the magnetic trap ofτ∼4...5s, depending on the Rb pressure.It ems reasonable to expect an order of magnitude gain if a parate MOT chamber is introduced and the pumping speed is im-proved.Thus,the atom chip materials are compatible with the vacuum level required for evaporative cooling or certain quantum manipulation experiments.
3.3Detection
The substrate is mounted upside down in the vacuum chamber,so that atoms are“hanging”below its surface. This enables time-of-flight imaging to measure the veloc-ity distribution.A probe beam for absorption imaging is
directed parallel to the surface along the y axis(figure 9).The shadow of the atom cloud is imaged onto a12 bit CCD camera by a multi-element zoom lens.Magni-fication is limited by the requirement to image the full ∼7mm length of the atomic conveyer belt.This in turn ts the resolution to23µm in object space,limited by the CCD pixel size.
After every absorption image,a reference image is taken without atoms.Dividing the intensities of both images and taking the logarithm yields the optical den-sities.All images shown in the following ction are ob-tained in this way.
4The trap loading procedure
One of the key issues of an atom chip experiment is to load cold atoms into microtraps which have much smaller volumes than usual magnetic traps and are lo-cated in clo proximity to a substrate surface.Our load-ing scheme relies on a mirror-MOT([6],e also[18]) for preparing cold atoms clo to a surface,and on a Fig.10The mirror-MOT.Left:perspective view indicating beam helicities of the di
烤玉米烤箱agonal beams which are incident on the mirror M,and the orientation of the quadrupole coils Q1and Q2.(The horizontal beams are not shown.)Right: Projections on the yz and xz planes.
field switching procedure in the MOT pha(from a “macroscopic”quadrupolefield to a“microscopic”one) to achieve reproducible trap loading without the need for precisionfield alignments.A schematic of the mirror-MOT is shown infig.10.Two of the six MOT lar beams are generated by reflection on the mirror,the re-sulting total lightfield is identical to that of a standard MOT.The loading procedure has been described in[6] and is summarized infigure11.We obtain up to6·106 atoms trapped in the MOT,limited by the small trap-ping beam1/e2diameter of8.5mm(which is itlf lim-ited by the substrate size,as MOT beams are reflected on it),by the total lar power of∼20mW,and by the rubidium pressure.We adjust this pressure to rather low the same order as the∼3·10−10mbar background pressure,in order to achieve long magnetic trapping times.A dispenr mounted above the sub-strate rves as thermal rubidium source and allows for relatively fast pressure adjustments.
5Demonstration of fundamental microtrap potentials
In Sect.2we have described how simple conductor pat-terns can be ud as“modules”,which can be
combined to form complex potentials.Some of the modules cre-ate potentials which are uful by themlves,and we start our prentation of experimental results with two of the:First,a conductor cross is ud to create an IP trap in which the long axis and the atomic polarization can be rotated about the conductor interction.In the cond experiment,a long,“Z”shaped conductor(which approximates the“H”shape of Sect.2)creates a very elongated IP trap,in which the atoms are strongly con-fined transversally and move almost freely about∼7mm along the long axis.
All experiments are carried out with the conductor pattern offigure12[6,11].The variety of basic conductor shapes which it contains makes it versatile enough for a large number of different experiments.
